In many applications, a metal structure or substrate may become corroded or oxidized when exposed to a particular atmosphere. Precautions can be taken to prevent such corrosion, but generally it occurs if a particular metal is exposed to a particular environment for an extended period of time. Many applications exist, such as automotive bodies and frames and aerospace application, where metal structures of vehicles are constantly exposed to extremely corrosive atmospheric conditions.
One mechanism of metallic corrosion is a galvanic reaction between the metal and the environment surrounding the metal. For example, oxygen in the atmosphere oxidizes the metal through a transfer of electrons from the metal to the oxygen at electrocatalytic sites on the metal surface and subsequent combination of the resulting metal cation with the oxygen anion to form a non-structural metal oxide corrosion product. In particular, water vapor acts as the electrolyte allowing oxygen to react with the metal. It has been proposed that conducting polymers can provide a pacification of this reaction by creating an anodic pacification. A coating is placed over the metallic substrate such that no reaction may occur unless there is an imperfection in the coating. When an imperfection occurs and the electrode reaction begins the conducting polymer acts as a cathode so as to supply a low but sufficient current to form a protective metal oxide film on the surface of the metal within the exposed defect.
Other films or coatings have been used which include hexavalent chromium, also referred to as chromate. In these coatings, the chromate acts as an inhibitor since it is water-soluble and reacts with the metal to form a barrier layer composed of the metal oxide and Cr2O3. In this reaction the metal is oxidized (looses electrons) and the chromate is reduced (gains electrons to go from the six valent chromate to the three valent Cr2O3). Preferably, the coating retains a portion of the chromate that can be released to protect the metal at imperfections in the film.
In this way, when an imperfection occurs and the electrode reaction is initiated, the chromate remaining in the film may migrate through the film and block the corrosion reaction with the atmosphere. In particular, the chromate moves into the imperfection of the coating to react with the metallic substrate thereby forming a protective layer. Nevertheless, a slow release of the chromate over time can reduce its availability to be released at the appropriate time. Although chromate is useful in this application, chromate may be toxic if ingested in sufficient amounts in a living organism. Therefore, strict and costly standards must be adhered to when using and disposing of the materials coated with the chromate.
Therefore, it is desirable to provide a coating which provides the blocking effects of chromate, but which is substantially not harmful to living organisms. In addition, it is desirable to provide a coating which produces an active or “smart” inhibition of the galvanic reaction created between a metallic substrate and the atmosphere when an imperfection in the coating occurs. It is desired to produce a coating which provides substantial inhibition to corrosion of a metallic substrate by the release of a blocking or inhibiting constituent into the defect to stop the corrosion of the metallic substrate.